Observing Apparent Nonuniform Sensitivity Enhancements in Dynamic Nuclear Polarization Solid-State NMR Spectra of Polymers

International audienceHigh-field dynamic nuclear polarization (DNP)may enhance the sensitivity of solid-state NMR experimentson a wide range of systems, including synthetic polymers,owing to the transfer of electron spin polarization from radicalsto nuclei upon microwave irradiation (usually at cryogenictemperatures). Provided that the radicals are homogeneouslydispersed in the sample, a uniform DNP enhancement isexpected for all the signals in the 13C cross-polarization magicangle spinning (CPMAS) spectrum. Here, we show that, in thecase of methyl group containing polymers, a change in thecross-polarization (CP) dynamics induced by the moderate increase in sample temperature due to microwave irradiation maylead to the observation of apparent nonuniform enhancements in the DNP-enhanced 13C CPMAS spectra. This peculiarbehavior should be accounted for when measuring 13C CP DNP enhancements on polymer materials, especially forheterogeneous polymer samples (for which truly nonuniform DNP enhancements could potentially be detected), or whenquantitative results are sought